Apparatus and method for providing fluid for immersion lithography

ABSTRACT

A liquid immersion lithography apparatus includes a projection system having a final optical element via which a substrate is exposed to an exposure beam through an immersion liquid located between the final optical element and the substrate, and a nozzle member. The nozzle member has a first opening on one side of the final optical element and from which the immersion liquid is supplied, a second opening on a second side of the final optical element and from which the immersion liquid is collected, and a liquid recovery portion that surrounds a path of the exposure beam and from which the immersion liquid is collected. During exposure, an upper surface of the substrate faces the liquid recovery portion, and the immersion liquid is supplied from the first opening while the immersion liquid supplied from the first opening is recovered from the second opening and the liquid recovery portion.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a divisional of U.S. patent application No. 12/461,243 filedAug. 5, 2009, which in turn is a continuation of U.S. patent applicationNo. 11/790,233 filed Apr. 24, 2007 (now abandoned), which is adivisional of U.S. patent application No. 11/362,833 filed Feb. 28, 2006(now U.S. Pat. No. 7,292,313), which is a continuation of InternationalApplication No. PCT/US2004/022915 filed Jul. 16, 2004, which claims thebenefit of U.S. Provisional Patent Application No. 60/500,312 filed Sep.3, 2003, and U.S. Provisional Patent Application No. 60/541,329 filedFeb. 2, 2004. The disclosures of these applications are incorporatedherein by reference in their entireties.

BACKGROUND

The invention relates generally to systems and methods for providingfluid for immersion lithography and, more particularly, for controllingthe fluid flow and pressure to provide stable conditions for immersionlithography.

An exposure apparatus is one type of precision assembly that is commonlyused to transfer images from a reticle onto a semiconductor wafer duringsemiconductor processing. A typical exposure apparatus includes anillumination source, a reticle stage assembly that retains a reticle, anoptical assembly, a wafer stage assembly that retains a semiconductorwafer, a measurement system, and a control system. The resist coatedwafer is placed in the path of the radiation emanating from a patternedmask and exposed by the radiation. When the resist is developed, themask pattern is transferred onto the wafer. In microscopy, extremeultraviolet (EUV) radiation is transmitted through a thin specimen to aresist covered plate. When the resist is developed, a topographic shaperelating to the specimen structure is left.

Immersion lithography is a technique that can enhance the resolution ofprojection lithography by permitting exposures with a numerical aperture(NA) greater than one, which is the theoretical maximum for conventional“dry” systems. By filling the space between the final optical elementand the resist-coated target (i.e., wafer), immersion lithographypermits exposure with light that would otherwise be totally internallyreflected at an optic-air interface. Numerical apertures as high as theindex of the immersion liquid (or of the resist or lens material,whichever is least) are possible. Liquid immersion also increases thewafer depth of focus, i.e., the tolerable error in the vertical positionof the wafer, by the index of the immersion liquid compared to a drysystem with the same numerical aperture. Immersion lithography thus hasthe potential to provide resolution enhancement equivalent to the shiftfrom 248 to 193 nm. Unlike a shift in the exposure wavelength, however,the adoption of immersion would not require the development of new lightsources, optical materials, or coatings, and should allow the use of thesame or similar resists as conventional lithography at the samewavelength. In an immersion system where only the final optical elementand its housing and the wafer (and perhaps the stage as well) are incontact with the immersion fluid, much of the technology and designdeveloped for conventional tools in areas such as contamination control,carry over directly to immersion lithography.

One of the challenges of immersion lithography is to design a system fordelivery and recovery of a fluid, such as water, between the finaloptical element and the wafer, so as to provide a stable condition forimmersion lithography.

SUMMARY

Embodiments of the invention are directed to systems and methods ofcontrolling the fluid flow and pressure to provide stable conditions forimmersion lithography. A fluid is provided in a space between the lensand the substrate during the immersion lithography process. Fluid issupplied to the space and is recovered from the space through a porousmember in fluidic communication with the space. Maintaining the pressurein the porous member under the bubble point of the porous member caneliminate noise created by mixing air with the fluid during fluidrecovery. The bubble point is a characteristic of the porous member thatdepends on the size of the holes in the porous member (the largest hole)and the contact angle that the fluid forms with the porous member (as aparameter based on the property of the porous material and the propertyof the fluid). Because the bubble point is typically a very lowpressure, the control of this low pressure becomes an important issue.

An aspect of the invention is directed to a method of recovering a fluidfrom a space between a lens and a substrate in an immersion lithographysystem. The method includes drawing the fluid from the space via arecovery flow line through a porous member and maintaining a pressure ofthe fluid in the porous member below a bubble point of the porous memberduring drawing of the fluid from the space.

In some embodiments, maintaining the pressure is accomplished byproviding an overflow container kept at a preset pressure and directingthe fluid drawn from the space through the porous member via therecovery flow line to the overflow container. Maintaining the pressurecan further include siphoning the fluid from the overflow container to acollection tank. The fluid is siphoned down by gravity to the collectiontank disposed below the overflow container. In other embodiments,maintaining the pressure includes providing a fluid level buffer,drawing the fluid from the space via a buffer flow line through theporous member to the fluid level buffer, sensing a pressure or a fluidlevel at the fluid level buffer, and controlling the fluid flow drawnfrom the space via the recovery flow line through the porous memberbased on the sensed pressure or fluid level at the fluid level buffer.Controlling the fluid flow can include controlling a variable valvedisposed in the recovery flow line downstream of the porous member. Instill other embodiments, maintaining the pressure includes providing afluid level buffer, drawing the fluid from the space via a buffer flowline through the porous member to the fluid level buffer, sensing apressure or a fluid level at the fluid level buffer, and controlling avacuum pressure at an outlet of the recovery flow line through theporous member based on the sensed pressure or fluid level at the fluidlevel buffer. Controlling the vacuum pressure can include controlling avacuum regulator in a collection tank at the outlet of the recovery flowline.

In accordance with another aspect of the invention, an apparatus forrecovering a fluid from a space between a lens and a substrate in animmersion lithography system includes an inner part that includes a lensopening to accommodate a portion of the lens and to position the lensapart from the substrate separated by the space to receive a fluid inthe space between the lens and the substrate. An outer part is disposedaround the inner part, and includes a porous member fluidicly coupledwith the space and with a fluid recovery outlet to draw fluid from thespace via the porous member to the fluid recovery outlet. A pressurecontrol system is fluidicly coupled with the porous member to maintain apressure at the surface of the porous member below a bubble point of theporous member during drawing of the fluid from the space via the porousmember.

In some embodiments, the pressure control system includes an overflowcontainer fluidicly coupled with the porous member and a vacuumregulator that regulates a pressure in the overflow container. Acollection tank is fluidicly coupled to and disposed below the overflowcontainer. In other embodiments, the pressure control system includes afluid level buffer fluidicly coupled with the porous member, a sensorthat senses a pressure or a fluid level at the fluid level buffer and acontroller that adjusts a flow rate of the fluid drawn from the spacethrough the fluid recovery outlet, based on a sensor signal output fromthe sensor, to maintain a pressure at the surface of the porous memberbelow a bubble point of the porous member during drawing of the fluidfrom the space via the porous member. The pressure control system caninclude a valve disposed downstream of the fluid recovery outlet, andthe controller controls the valve to adjust the flow rate of the fluiddrawn from the space through the fluid recovery outlet. In still otherembodiments, the pressure control system includes a collection tankfluidicly coupled to the fluid recovery outlet and a controllable vacuumregulator that regulates a pressure in the collection tank. Thecontroller controls the controllable vacuum regulator to adjust the flowrate of the fluid drawn from the space through the fluid recovery outletto the collection tank by controlling the pressure in the collectiontank.

In specific embodiments, the inner part is spaced from the outer part byan intermediate spacing. The inner part includes an inner cavity forminga part of the spacing between the lens and the substrate, and the innerpart includes apertures disposed above the inner cavity for at least oneof introducing fluid into and drawing fluid from the inner cavity. Theinner part includes apertures disposed on opposite sides of the lensopening for introducing fluid into the inner cavity. The inner partincludes a pair of buffer slots disposed on opposite sides of the lensopening in a direction of scan of the immersion lithography system. Theinner part includes purge holes and each of the pair of buffer slots isfluidicly coupled to at least one of the purge holes. The porous memberis selected from the group consisting of a mesh, a porous material, anda member having etched holes therein.

In accordance with another aspect of the invention, an apparatusincludes an optical projection system having a last optical element andthat projects an image onto a workpiece, and a stage that supports theworkpiece adjacent to the optical projection system when the image isbeing projected onto the workpiece. A gap is provided between the lastoptical element and the workpiece and is filled with an immersion fluid.A porous material is positioned adjacent to the gap and recovers fluidexiting the gap. A control system maintains a pressure on the porousmaterial. The pressure is at or below the bubble point of the porousmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements and in which:

FIG. 1 is a simplified elevational view schematically illustrating animmersion lithography system according to an embodiment of theinvention;

FIG. 2 is a perspective view of a nozzle for fluid delivery and recoveryfor immersion lithography according to one embodiment of the invention;

FIG. 3 is a simplified cross-sectional view of the nozzle of FIG. 2;

FIG. 4 is a cross-sectional view of the inner part of the nozzle of FIG.2;

FIG. 5 is a simplified cross-sectional view of the nozzle according toanother embodiment;

FIG. 6 is a simplified view schematically illustrating a pressurecontrol system for fluid recovery in an immersion lithography systemaccording to one embodiment of the invention;

FIG. 7 is a simplified view schematically illustrating a pressurecontrol system for fluid recovery in an immersion lithography systemaccording to another embodiment of the invention;

FIG. 8 is a simplified view schematically illustrating a pressurecontrol system for fluid recovery in an immersion lithography systemaccording to another embodiment of the invention; and

FIG. 9 is a simplified view schematically illustrating a pressurecontrol system for fluid recovery in an immersion lithography systemwith water stagnation prevention according to another embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an immersion lithography system 10 including a reticlestage 12 on which a reticle is supported, a projection lens 14, and awafer 16 supported on a wafer stage 18. An immersion apparatus 20, whichis sometimes referred to herein as a showerhead or a nozzle, is disposedaround the final optical element 22 of the projection lens 14 to provideand recover a fluid, which may be a liquid such as water or a gas,between the final optical element 22 and the wafer 16. In the presentembodiment, the immersion lithography system 10 is a scanninglithography system in which the reticle and the wafer 16 are movedsynchronously in respective scanning directions during a scanningexposure.

FIGS. 2 and 3 show the apparatus or nozzle 20 for delivery and recoveryof the fluid between the final optical element 22 and the wafer 16 forimmersion lithography. FIG. 2 shows the bottom perspective view of thenozzle 20, which includes an outer part 30 and an inner part 32. Theinner part 32 defines an inner cavity 34 to receive the fluid betweenthe final optical element 22 and the wafer 16. The inner part 32includes apertures 38 for fluid flow into and out of the inner cavity34. As seen in FIG. 2, there are apertures 38 disposed on both sides ofthe final optical element 22. The inner part 32 has a flat portion 33surrounding the inner cavity 34. The flat portion 33 is substantiallyparallel to the wafer 16. The distance D1 between the end surface of thefinal optical element 22 and the wafer 16 is greater than the distanceD2 between the flat portion 33 and the wafer 16. The distance D1 couldbe 1.0-5.0 mm, and the distance D2 could be 0.5-2.0 mm. In anotherembodiment, the distance D1 is substantially equal to the distance D2.The inner part 32 further includes a pair of buffers or buffer slots 40with purge holes 42 (see FIG. 4). The buffers 40 are arranged at or nearthe flat portion 33. The buffers 40 are disposed on opposite sides ofthe final optical element 22. A cross-sectional view of the inner part32 in the direction of scan 44 is illustrated in FIG. 4.

The outer part 30 is spaced from the inner part 32 by an intermediatespacing or groove 48, which may be referred to as an atmospheric groove.The outer part 30 includes one or more fluid recovery openings 50disposed on opposite sides of the final optical element 22. A porousmember 51 is disposed in a slot or outer cavity 53 that extends aroundthe inner part 32 and fluidicly communicates with the pair of fluidrecovery openings 50. The porous member 51 may be a mesh or may beformed of a porous material having holes typically in the size range ofabout 50-200 microns. For example, the porous member 51 may be a wiremesh including woven pieces or layers of material made of metal,plastic, or the like, a porous metal, a porous glass, a porous plastic,a porous ceramic, or a sheet of material having chemically etched holes(e.g., by photo-etching). The porous member 51 is desirably hydrophilic.The outer part 30 further includes a fluid buffer outlet 56 and a fluidrecovery outlet 58. In another embodiment of the nozzle 20′ as seen inFIG. 5, the inner part 32 does not contact or form a seal with the finaloptical element 22, but is spaced from the final optical element 22. Thegap prevents nozzle vibrations from being transmitted to the finaloptical element 22. However, the gap may allow the fluid to be exposedto air.

One feature of the nozzle 20 is that it is made in two pieces, namely,the outer part 30 and the inner part 32. The inner part 32 keeps thefluid between the lens and the wafer surface, and the outer part 30 ismainly provided for fluid recovery. Vibration might be introduced duringfluid recovery from the outer part 30 through the porous member 51 tothe other components of the lithography system, including the inner part32 which may be used to direct an autofocus beam to the wafer 16. Adamping material can be mounted between the outer part 30 and themounting piece to which the outer part 30 is mounted to minimize thetransmission of vibration from the outer part 30. In addition, the outerpart 30 that includes the porous member may be prone to contaminationand thus needs to be replaced for maintenance. Making the outer part 30as a separate part facilitates easier maintenance. It can also minimizereadjustment and recalibration time after replacement of the outer partas opposed to replacing the entire nozzle 20. Manufacturability of thenozzle 20 can also be improved if the nozzle 20 is made in two separateparts. It is understood that the nozzle 20 may be made of a single piecein alternative embodiments.

Another feature of the nozzle 20 is the atmospheric groove 48 betweenthe inner part 32 and the outer part 30. This atmospheric groove 48functions as a breaking edge to prevent fluid in the inner part 32 frombeing drawn out by the porous member 51 on the outer part 30 if thefluid recovery rate is faster than the fluid supply rate. In thesituation when there is no breaking edge, a balance between the fluidrecovery rate and the fluid supply rate has to be maintained so thatfluid can be kept within the inner part 32 at all times during scanning.Having the atmospheric groove 48 allows the recovery rate to be set at amaximum to minimize fluid leakage out of the outer part 30 duringscanning. The atmospheric groove 48 also acts as a buffer for fluid togo in and out during scanning, minimizing water supply and recoveryrequirements.

In the process of immersion lithography, a fluid is to be filled betweenthe projection lens 14 and the wafer 16 from a dry state and, at othertimes, the fluid is to be recovered. For example, in the beginning ofexposure of a new wafer, the fluid is to completely fill the innercavity 34 of the inner part 32 before starting exposure. During thisprocess, ideally no air bubbles can exist between the projection lens 14and wafer 16 or other optical paths such as the auto focus beam. Thefluid supply in the inner cavity of the inner part 32 is designed to beat the highest point in the cavity (via apertures 38) so that the fluidis filled from top down, allowing air bubbles to be pushed out of theinner cavity during the filling process. The fluid desirably isinitially supplied from one side in this embodiment (the set ofapertures 38 on one side), so that the fluid is filled from one side tothe other, again allowing air bubbles to be pushed out to avoid trappingair therein. Other arrangements are also possible, as long as the fluidis being filled from the inside out.

On occasion, the fluid has to be fully recovered from the inner cavityof the inner part 32. In FIG. 4, there are small holes 42 in each of thebuffers 40 in the inner cavity. These holes 42 are provided for fastfluid recovery or fluid purge when the fluid has to be fully recovered.Sucking the fluid out from these holes 42 using high vacuum with thecombination of some movement in the wafer stage 18 allows all the fluidto be recovered within a reasonable time.

The inner part 32 has two groups or rows of holes 38 for supplying orrecovering the fluid. Each row can be independently controlled to eithersupply or recover the fluid. In the case where both rows are chosen forfluid supply, all the fluid is recovered through the porous member 51 inthe outer part 30. Because both rows are supplying fluid, a pressure canbuild up in the inner cavity causing deformation of the final opticalelement 22 of the projection lens 14 or the wafer 16 or both. The fluidflow across the final optical element 22 may also be limited, and thusthe temperature of the fluid between the final optical element 22 andthe wafer 16 may eventually rise, causing adverse effects. On the otherhand, if one row is chosen for supply and the other for recovery, afluid flow will be driven across the final optical element 22,minimizing temperature rise. It can also reduce the pressure otherwisecreated by supplying fluid from both rows. In this case, less fluidneeds to be recovered through the porous member 51, lowering the fluidrecovery requirement in the porous member. In other nozzleconfigurations, multiple fluid supplies and recoveries may be providedso as to optimize the performance.

During scanning motion of the wafer stage 18 (in the direction of scan44 in FIG. 2), the fluid may be dragged in and out of the inner cavityof the inner part 32. When the fluid is dragged out, it is recoveredthrough the porous member 51 in the outer part 30. When the wafer stage18 is moved in the opposite direction, air may be dragged into the innercavity of the inner part 32. During this time, the fluid in the buffers40, as well as the fluid supplied from within the inner cavity, helps torefill the fluid that is dragged along the scanning direction,preventing air from getting into the inner cavity. The buffers 40 andthe porous member 51 work together to minimize fluid leaking out fromthe outer part 30, and air dragging into the inner cavity of the innerpart 32 during scanning motion of the wafer stage 18.

Recovering fluid through the porous member 51 by maintaining thepressure in the porous member 51 under the bubble point can eliminatenoise created by mixing air with the fluid during fluid recovery. Thebubble point is a characteristic of the porous member 51 that depends onthe size of the holes in the porous member 51 (the largest hole) and thecontact angle that the fluid forms with the porous member 51 (as aparameter based on the property of the porous material and the propertyof the fluid). Due to the fact that the bubble point is typically a verylow pressure (e.g., about 1000 pascal), the control of this low pressurebecomes an important issue. FIGS. 6-7 illustrate three specific ways ofmaintaining the pressure below the bubble point during fluid recovery.

In the pressure control system 100 of FIG. 6, a pressure under bubblepoint is maintained at the surface of the porous member 51 using avacuum regulator 102 with the assistance of an overflow container ortank 104 fluidicly coupled to the porous member 51 by a recovery flowline 106 (which is connected to the fluid buffer outlet 56). Thepressure at the surface of the porous member 51 is equal to the pressuremaintained by the vacuum regulator 102 subtracting the pressure createdby the height of the fluid above the porous member 51 Maintaining aconstant height of fluid above the porous member 51 using the overflowtank 104 allows easy control of the pressure at the surface of theporous member 51. The fluid that is recovered through the porous member51 will overflow and be siphoned down along a siphon line 108 to acollection tank 110, which is disposed below the overflow tank 104. Anoptional flow path 112 is connected between the overflow tank 104 andthe collection tank 110 to assist in equalizing the pressure between theoverflow tank 104 and the collection tank 110 and facilitate flow alongthe siphon line 108. One feature of this pressure control system 100 isthat it is a passive system without the necessity of control.

In the pressure control system 120 of FIG. 7, the pressure at thesurface of the porous member 51 is maintained below the bubble pointusing a vacuum regulator 122 at a fluid level buffer 124 which isfluidicly coupled with the porous member 51 by a buffer flow line 126(which is connected to the fluid buffer outlet 56). A pressuretransducer or a water level sensor 128 is used to measure the pressureor fluid level at the fluid level buffer 124. The sensor signal is thenused for feedback control 130 to a valve 132 that is disposed in arecovery flow line 134 (which is connected to the fluid recovery outlet58) connected between the porous member 51 and a collection tank 136.The valve 132 may be any suitable valve, such as a proportional orvariable valve. The variable valve 132 is adjusted to control the fluidflow through the fluid recovery line 134 to the collection tank 136 tomaintain the pressure or fluid level of the fluid level buffer 124 at apreset value. The collection tank 136 is under a relatively highervacuum controlled by a high vacuum regulator 138 for fluid recovery. Inthis fluid control system 120, no overflow tank is needed and thecollection tank 136 can be placed anywhere in the system and need not bedisposed below an overflow tank. An on/off valve 140 is desirablyprovided in the fluid recovery line 134 and is switched off when fluidrecovery is not required.

In FIG. 8, the pressure control system 160 is similar to the system 120of FIG. 7, and like reference characters are used for like parts.Instead of using the valve 132 for the feedback control of fluidrecovery, this system 160 employs a controllable vacuum regulator 162for the feedback control of fluid recovery. The vacuum regulator 162 istypically electronically controllable to adjust the vacuum pressure inthe collection tank 136 based on the sensor signal from the pressuretransducer or a water level sensor 128. The vacuum regulator 162 isadjusted to control the fluid flow through the fluid recovery line 134to the collection tank 136 to maintain the pressure or fluid level ofthe fluid level buffer 124 at a preset value. The on/off valve 140 inthe fluid recovery line 134 is switched off when fluid recovery is notrequired.

FIG. 9 shows a pressure control system for fluid recovery in animmersion lithography system with water stagnation prevention accordingto another embodiment of the invention. The pressure control system 180is similar to the system 120 of FIG. 7 having the same components withthe same reference characters. In addition, the fluid level buffer 124is fluidicly coupled with a fluid supply or fluid recovery 182 to supplyfluid to or recover fluid from the fluid level buffer 124 to preventstagnation. An optional pump or a similar moving part may be used toinduce flow between the fluid level buffer 124 and the fluid supply orfluid recovery 182. There is a possibility of bacteria/fungus growth instagnated water or fluid over time. Under normal operation, the water atthe fluid level buffer 124 is stagnated because water recovered from themesh 51 will go through the small tube at the mesh level to thecollection tank 136. By inducing flow into or out of the fluid levelbuffer 124 during normal operation, the bacteria/fungus growth problemcan be prevented.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reviewing the above description. Thescope of the invention should not be limited to the above description.

Also, the present invention could be applied to Twin-Stage-TypeLithography Systems. Twin-Stage-Type Lithography Systems, for example,are disclosed in U.S. Pat. No. 6,262,796 and U.S. Pat. No. 6,341,007,the disclosures of which are incorporated herein by reference in theirentireties.

What is claimed is:
 1. A liquid immersion lithography apparatuscomprising: a projection system having a final optical element via whicha substrate is exposed to an exposure beam through an immersion liquidlocated between the final optical element and the substrate; and anozzle member having: a first opening disposed on one side of the finaloptical element and from which the immersion liquid is supplied; asecond opening disposed on a second side of the final optical elementand from which the immersion liquid is collected; and a liquid recoveryportion disposed such that the liquid recovery portion surrounds a pathof the exposure beam and from which the immersion liquid is collected,wherein the nozzle member is spaced from the final optical element,during the exposure, an upper surface of the substrate faces the liquidrecovery portion, and during the exposure, the immersion liquid issupplied from the first opening while the immersion liquid supplied fromthe first opening is recovered from the second opening and the liquidrecovery portion.
 2. The apparatus of claim 1, wherein: the nozzlemember has an undersurface disposed such that the undersurface surroundsthe path of the exposure beam, and the first and second openings of thenozzle member are located higher than the undersurface of the nozzlemember.
 3. The apparatus of claim 2, wherein the first and secondopenings are located at a same height.
 4. The apparatus of claim 2,wherein the nozzle member has a recess arranged such that the recess issurrounded by the undersurface.
 5. The apparatus of claim 4, wherein theundersurface is located between the recess and the recovery portion. 6.The apparatus of claim 2, wherein the liquid recovery portion isarranged such that the liquid recovery portion surrounds theundersurface.
 7. The apparatus of claim 2, wherein the undersurface andthe liquid recovery portion are disposed such that an upper surface ofthe substrate faces the undersurface and the liquid recovery portionduring the exposure.
 8. The apparatus of claim 2, wherein the immersionliquid is supplied from the first opening and the immersion liquid isrecovered from the second opening such that the immersion liquid flowsacross the final optical element.
 9. The apparatus of claim 1, whereinthe immersion liquid is supplied from the first opening and theimmersion liquid is recovered from the second opening such that theimmersion liquid flows across the final optical element.
 10. Theapparatus of claim 9, wherein temperature rise of the immersion liquidis prevented.